The Mongols, in about a century, managed to kill about 10% of the world’s population.
No other group has ever come close to that record.
The Nazis, for example, killed perhaps 20M people, and the Commies perhaps 125M, out of a population of perhaps 2.5B.
That’s 0.8% for the Nazis and 5% for the commies.
So Ghenghis and his grandson win the Hell-bent award.